1. Field of the Invention
The present invention relates to a valve timing adjusting device for changing the opening/closing timing (hereinafter referred to as the "valve timing") to open/close an intake valve and/or an exhaust valve of an internal combustion engine (hereinafter referred to as the "engine") in accordance with a drive condition.
2. Related Art
There has been known in the art a vane type valve timing adjusting device in which a cam shaft is driven by driving force transmitting mechanism such as a chain sprocket rotating in synchronism with the crankshaft of an engine so that the valve timing of at least one of the intake valve and the exhaust valve is controlled with a phase difference resulting from the relative rotations between the driving force transmitting mechanism and the cam shaft.
In this vane type valve timing adjusting device, a vane rotating with the cam shaft is housed in a housing rotating with the driving force transmitting mechanism. By adjusting the relative rotation phase difference of the vane to the housing hydraulically, moreover, the cam shaft and the driving force transmitting mechanism are rotated relatively to each other to adjust the valve timing of at least one of the intake valve and the exhaust valve in accordance with the drive condition of the engine.
The phase control valve timing adjusting device for controlling the valve timing of the engine valve aims at improving the stability and fuel efficiency of the engine or reducing the exhaust emission. At a light load condition of the engine of this kind, the intake air amount is so small as to make it desirable to reduce the residual exhaust gas, as might otherwise deteriorate the combustion, in the cylinder of the engine.
For a time period (or an overlap period) for which the intake valve and the exhaust valve are simultaneously open, a negative pressure is established on the intake side by the throttle, whereas a positive pressure prevails in the exhaust side. This may invite the case in which the exhaust gas is blown back to the intake side to deteriorate the combustion or to invite a misfire. Therefore, it is demanded to close the exhaust valve early and to open the intake valve late.
By retarding the timing for closing the intake valve, on the other hand, the pumping loss can be reduced to improve the fuel efficiency. At the idling time and the starting time, therefore, the control has to be made in the fundamental phase where the exhaust valve is closed early and where the intake valve is opened late. Here, the condition of this fundamental phase on the intake side defines the most retarded angle, and the condition on the exhaust side defines the most advanced angle.
At an intermediate or heavier load of the engine, however, the EGR ratio is controlled to reduce the pumping loss by the internal EGR thereby to improve the fuel economy and reduce the exhaust emission. This makes it necessary to advance the valve opening timing on the intake side or to retard the valve opening timing on the exhaust side. In short, the intake valve is controlled in the advancing direction whereas the exhaust valve is controlled in the retarding direction.
At the heaviest load of the engine, moreover, a large amount of air has to be introduced into the cylinder of the engine. This makes it necessary to close the intake valve early in the low speed range thereby preventing the reverse flow into the manifold and to make use of the inertia of the air in the high speed range thereby closing the intake valve late.
On the exhaust side, on the other hand, the exhaust valve is controlled to the phase capable of making the maximum use of exhaust pulsations so that the advanced angle has to be controlled to the maximum if the exhaust pulsations cannot be used. In short, on the exhaust side, the exhaust valve has to be controlled from the light load of the engine in the retarding direction from the most advanced position and again in the advancing direction in accordance with the load.
At this time the intake/exhaust valve can desirably be controlled quickly to the demanded phase if the drive condition changes. When it is impossible to control the intake/exhaust valve, however, there may occur a problem such as the misfire or the combustion instability of the engine.
Usually, the hydraulic pump of the engine is driven by the crankshaft. As a result, however, the flow amount of the oil to be discharged varies according to the rotation speed of the engine, and it decreases at a low rotation speed of the engine. As a result, the oil pressure may be decreased by the leakage and the drop of the viscosity especially at a high oil temperature, and the actuator may not operate. At this time, the intake side is retarded by the driving torque of the cam shaft so that it can take the fundamental phase. When an actuator having the same hydraulic piston area as that of the intake side, however, the exhaust side may not be controlled to the fundamental position, and the residual gas in the cylinder of the engine may increase to cause the misfire or stop the engine.
To solve the above problem, a valve timing adjusting device disclosed in JP-A-9-264110 moves the intake side to the retarded position or moves the discharge side to the advanced position by the biasing force of a torsion spring.
However, the torsion spring is structurally required to construct a spring around the whole circumference of the cam shaft. This requirement makes it necessary to form a housing space for housing the torsion spring, around the whole circumference of the cam shaft in the axial direction.
The vane type phase variable actuator generates an operating torque by controlling the oil pressure between the front and back of the vane members. If the aforementioned housing space is formed around the whole circumference of the cam shaft in the axial direction, therefore, the hydraulic chambers at the front and back of the vane members may be connected to fail to generate a pressure necessary for the operation.
In order to prevent the connection between the hydraulic chambers at the front and the back of the vane members, it is necessary to set the internal diameter of the hydraulic chamber, that is, the internal diameter of the vane members larger than an external diameter of the torsion spring. In short, the area across the vane members has to be retained to retain the oil pressure for rocking the vane members.
If the internal diameter of the vane members is set larger than the external diameter of the torsion spring, however, the external diameter of the hydraulic chamber, that is, the external diameter of the vane members has to be made relatively large. Accordingly, the actuator becomes bigger. This enlarged structure raises problems that the valve timing adjusting device is so raised in its weight and manufacturing cost as to make it difficult to mount it on the engine.
If the area across the vane members is enlarged by increasing the number of vane members so as to make the external diameter of the hydraulic chamber relatively small, on the other hand, there arises a problem that the number of parts increases to raise the manufacturing cost. Another problem is that the increase in the number of vane members reduces the rocking angle of the vane members so that the rocking angle of the vane members necessary for improving the engine performance cannot be achieved to lower the engine performance.
Further, a valve timing adjusting device disclosed in JP-A-10-68306 moves the discharge side to the advanced position by the biasing force of a torsion spring. Accordingly, when a vane type phase variable actuator is used, the response in the advancing direction is improved. However, the response in the retarding direction is compromised comparing to the one without the torsion spring.
Furthermore, when the vane is held at a predetermined position, hydraulic fluid, having higher pressure than that of hydraulic fluid to be supplied to the retard angle hydraulic chamber, is supplied to the advance angle hydraulic chamber. Accordingly, the pressure difference between the advance angle hydraulic chamber and the retard angle hydraulic chamber increases, and an oil leakage may occur therebetween.
Further, the area of the vane has to be increased in order to perform the phase control with substantially low hydraulic pressure. Accordingly, the actuator is increased in size, and the valve timing adjusting device is increased in weight and manufacturing cost. Thus, it may be difficult to mount it on the engine.